§ 73.186 Establishment of effective field at one kilometer.
(a) Section 73.189 provides that certain minimum field strengths are
acceptable in lieu of the required minimum physical heights of the
antennas proper. Also, in other situations, it may be necessary to
determine the effective field. The following requirements shall govern
the taking and submission of data on the field strength produced:
(1) Beginning as near to the antenna as possible without including the
induction field and to provide for the fact that a broadcast antenna is
not a point source of radiation (not less than one wave length or 5
times the vertical height in the case of a single element, i.e. ,
nondirectional antenna or 10 times the spacing between the elements of
a directional antenna), measurements shall be made on six or more
radials, at intervals of approximately 0.2 kilometer up to 3 kilometers
from the antenna, at intervals of approximately one kilometer from 3
kilometers to 5 kilometers from the antenna, at intervals of
approximately 2 kilometers from 5 kilometers to 15 kilometers from the
antenna, and a few additional measurements if needed at greater
distances from the antenna. Where the antenna is rurally located and
unobstructed measurements can be made, there shall be at least 15
measurements on each radial. These shall include at least 7
measurements within 3 kilometers of the antenna. However, where the
antenna is located in a city where unobstructed measurements are
difficult to make, measurements shall be made on each radial at as many
unobstructed locations as possible, even though the intervals are
considerably less than stated above, particularly within 3 kilometers
of the antenna. In cases where it is not possible to obtain accurate
measurements at the closer distances (even out to 8 or 10 kilometers
due to the character of the intervening terrain), the measurements at
greater distances should be made at closer intervals.
(2) The data required by paragraph (a)(1) of this section should be
plotted for each radial in accordance with either of the two methods
set forth below:
(i) Using log-log coordinate paper, plot field strengths as ordinate
and distance as abscissa.
(ii) Using semi-log coordinate paper, plot field strength times
distance as ordinate on the log scale and distance as abscissa on the
linear scale.
(3) However, regardless of which of the methods in paragraph (a)(2) of
this section is employed, the proper curve to be drawn through the
points plotted shall be determined by comparison with the curves in
§ 73.184 as follows: Place the sheet on which the actual points have
been plotted over the appropriate Graph in § 73.184, hold to the light
if necessary and adjust until the curve most closely matching the
points is found. This curve should then be drawn on the sheet on which
the points were plotted, together with the inverse distance curve
corresponding to that curve. The field at 1 kilometer for the radial
concerned shall be the ordinate on the inverse distance curve at 1
kilometer.
(4) When all radials have been analyzed in accordance with paragraph
(a)(3) of this section, a curve shall be plotted on polar coordinate
paper from the fields obtained, which gives the inverse distance field
pattern at 1 kilometer. The radius of a circle, the area of which is
equal to the area bounded by this pattern, is the effective field. (See
§ 73.14.)
(5) The antenna power of the station shall be maintained at the
authorized level during all field measurements. The power determination
will be made using the direct method as described in § 73.51(a) with
instruments of acceptable accuracy specified in § 73.1215.
(b) Complete data taken in conjunction with the field strength
measurements shall be submitted to the Commission in affidavit form
including the following:
(1) Tabulation by number of each point of measurement to agree with the
maps required in paragraph (c) of this section, the date and time of
each measurement, the field strength (E), the distance from the antenna
(D) and the product of the field strength and distance (ED) (if data
for each radial are plotted on semilogarithmic paper, see paragraph
(a)(2)(ii) of this section) for each point of measurement.
(2) Description of method used to take field strength measurements.
(3) The family of theoretical curves used in determining the curve for
each radial properly identified by conductivity and dielectric
constants.
(4) The curves drawn for each radial and the field strength pattern.
(5) The antenna resistance at the operating frequency.
(6) Antenna current or currents maintained during field strength
measurements.
(c) Maps showing each measurement point numbered to agree with the
required tabulation shall be retained in the station records and shall
be available to the FCC upon request.
[ 28 FR 13574 , Dec. 14, 1963, as amended at 41 FR 44178 , Oct. 7, 1976;
46 FR 11995 , Feb. 12, 1981; 49 FR 49851 , Dec. 24, 1984; 50 FR 18843 ,
May 2, 1985; 50 FR 47055 , Nov. 14, 1985; 51 FR 2707 , Jan. 21, 1986; 52 FR 10570 , Apr. 2, 1987; 66 FR 20757 , Apr. 25, 2001]
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